It was raiden from MGS4. There was a small agreement with ubisoft and konami a few years before brotherhood where in MGS4, Snake had an Altair costume. You can see another hint of this agreement in MGS: Peace Walker when in a level (i forget which one) you can do a leap of faith into a hay bale which then unlocks the assassin box item.

Snake hides and sneaks around in cardboard boxes. In Peace Walker there were a few variations of these boxes with different effects, such as the tank box, love box, assassin box.

-Love box is used for only one special mission.
-Assassin box is that haybale in my above pic and it alows you to do a hidden takedown like in AC when the assassin is in the haybale and pulls the guard into it.
-Tank box works in multiplayer. If you and another player get in the box, you get the stealth aspect of the box while the second player is operating the tank gun on top of the box to take down guards from afar.

There is a mission which is a 'date' with the character Paz. You must do whatever you can in the 'date' to make her warm up to you. And if you can do that, you can use the love box near her and she will get into it with you and a few hearts will pop up above the box and you win the mission.